Re: [PATCH] mm: fix long time stall from mm_populate

[Minchan Kim <minchan@kernel.org>]

On Tue, Feb 11, 2020 at 04:23:23AM -0800, Matthew Wilcox wrote:
> On Mon, Feb 10, 2020 at 08:25:36PM -0800, Minchan Kim wrote:
> > On Mon, Feb 10, 2020 at 07:54:12PM -0800, Matthew Wilcox wrote:
> > > On Mon, Feb 10, 2020 at 07:50:04PM -0800, Minchan Kim wrote:
> > > > On Mon, Feb 10, 2020 at 05:10:21PM -0800, Matthew Wilcox wrote:
> > > > > On Mon, Feb 10, 2020 at 04:19:58PM -0800, Minchan Kim wrote:
> > > > > >       filemap_fault
> > > > > >         find a page form page(PG_uptodate|PG_readahead|PG_writeback)
> > > > > 
> > > > > Uh ... That shouldn't be possible.
> > > > 
> > > > Please see shrink_page_list. Vmscan uses PG_reclaim to accelerate
> > > > page reclaim when the writeback is done so the page will have both
> > > > flags at the same time and the PG reclaim could be regarded as
> > > > PG_readahead in fault conext.
> > > 
> > > What part of fault context can make that mistake?  The snippet I quoted
> > > below is from page_cache_async_readahead() where it will clearly not
> > > make that mistake.  There's a lot of code here; please don't presume I
> > > know all the areas you're talking about.
> > 
> > Sorry about being not clear. I am saying  filemap_fault ->
> > do_async_mmap_readahead
> > 
> > Let's assume the page is hit in page cache and vmf->flags is !FAULT_FLAG
> > TRIED so it calls do_async_mmap_readahead. Since the page has PG_reclaim
> > and PG_writeback by shrink_page_list, it goes to 
> > 
> > do_async_mmap_readahead
> >   if (PageReadahead(page))
> >     fpin = maybe_unlock_mmap_for_io();
> >     page_cache_async_readahead
> >       if (PageWriteback(page))
> >         return;
> >       ClearPageReadahead(page); <- doesn't reach here until the writeback is clear
> >       
> > So, mm_populate will repeat the loop until the writeback is done.
> > It's my just theory but didn't comfirm it by the testing.
> > If I miss something clear, let me know it.
> 
> Ah!  Surely the right way to fix this is ...

I'm not sure it's right fix. Actually, I wanted to remove PageWriteback check
in page_cache_async_readahead because I don't see corelation. Why couldn't we
do readahead if the marker page is PG_readahead|PG_writeback design PoV?
Only reason I can think of is it makes *a page* will be delayed for freeing
since we removed PG_reclaim bit, which would be over-optimization for me.

Other concern is isn't it's racy? IOW, page was !PG_writeback at the check below
in your snippet but it was under PG_writeback in page_cache_async_readahead and
then the IO was done before refault reaching the code again. It could be repeated
*theoretically* even though it's very hard to happen in real practice.
Thus, I think it would be better to remove PageWriteback check from
page_cache_async_readahead if we really want to go the approach.

However, page_cache_async_readahead has another condition to bail out: ra_pages
I think it's also racy with fadvise or shrinking the window size from other tasks.

That's why I thought second trial with non-fault retry logic from caller would fix
all potnetial issues all at once like page fault handler have done.

> 
> +++ b/mm/filemap.c
> @@ -2420,7 +2420,7 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
>                 return fpin;
>         if (ra->mmap_miss > 0)
>                 ra->mmap_miss--;
> -       if (PageReadahead(page)) {
> +       if (!PageWriteback(page) && PageReadahead(page)) {
>                 fpin = maybe_unlock_mmap_for_io(vmf, fpin);
>                 page_cache_async_readahead(mapping, ra, file,
>                                            page, offset, ra->ra_pages);
>